Natural gas recovered from underground reservoirs is usually compressed and cooled prior to storage and transport to the point of sale or use. Liquid is often found in natural gas streams and it is important to remove as much water or condensed hydrocarbons from the gas as possible in order to avoid damage to compressors and other process equipment. Liquid components are removed from natural gas streams by various means, for example using gas scrubbers and other forms of separator. It is desirable to measure the amount of liquid remaining in a gas stream after it has been treated in order to ensure as complete removal as possible.
The use of nucleonic gauges is common for measuring the bulk density of a process fluid for determining level or a density profile of a multiphase fluid for example. Such gauges typically measure the gamma radiation that passes through the fluid and which is detected by a detector. Gauges based on the transmission of gamma radiation are not, however, sufficiently sensitive to detect small changes in the density of a gas. The detection of small amounts of liquid entrained in a gas stream is of commercial importance in the gas supply industry for the reasons already discussed and yet the resulting changes in the gas density are typically too small for the existing gamma gauges to detect. WO06/067525 describes a method of detecting liquid in a gas stream by measuring changes in the bulk density of a gas stream using gamma radiation. The radiation source and detector are located outside a vessel such as a high-pressure gas pipeline and radiation passes from the source through the pipe to the detector. The radiation enters and exits the pipe walls through titanium dip tubes which are much less attenuating to the radiation than the material of the pipe walls. U.S. Pat. No. 6,389,908 describes an alternative method using gamma radiation, where the gamma is transmitted through a narrowed portion of a pipeline through beryllium windows inserted into the pipe walls. In order to increase the sensitivity of such a method, it is necessary to use less energetic radiation such as X-rays or beta particles. Beta gauges are currently used for measuring online the thickness of paper in paper mills. However, beta particles are of such low energies that they are unable to penetrate the dip tubes or beryllium windows used in the prior gauges. It is an object of the invention to provide a method for determining the bulk density of a gas stream which is suitable for estimating the amount of a liquid present in order to monitor the effectiveness of liquid removal processes.